Dry silver photosensitive or photothermographic compositions comprise an intimate mixture of a light-sensitive silver halide and another silver compound such as a silver salt of an organic acid, e.g. silver behenate or silver saccharine, which upon reduction gives a visible change and which is substantially light-insensitive. Such a mixture is usually prepared in suspension and the resulting dispersion spread as a layer on a suitable substrate to produce, for example, a sheet material. When dry, the layer is exposed to a light image and thereafter a reproduction of the image can be developed by heating the layer in the presence of a reducing agent for silver ions such as hydroquinone or certain substituted phenols.
It is because the exposure and development of the layer occur without using water, that these materials are often referred to as dry silver light-sensitive materials. Such materials, in which minor amounts of a photosensitive silver halide catalyst-progenitor are associated in catalytic proximity with major amounts of a heat sensitive oxidation-reduction image forming reaction mixture which mixture reacts more rapidly in the presence of the catalyst (silver) resulting upon exposure of the silver halide, are well known in the art.
It is believed that when the mixture is exposed to light a latent image is formed in the silver halide. Thereafter, the silver compound can be reduced by heating in the presence of the reducing agent, this reduction being catalyzed image-wise by the silver formed on the light exposed silver halide. By a suitable choice of temperature, the reduction of the silver compound can be catalyzed in the light exposed areas to give a visible darkening while any slight reduction which occurs in the non-light exposed areas is insufficient to give a marked visible change. Because the silver halide acts as a catalyst progenitor, very small amounts of it can suffice, e.g. 0.1 to 10% by weight of the mixture. However, large amounts, e.g. up to 15 or even 20% may be desirable in some circumstances.
In order to improve the sharpness or definition of photographic images a dye known as an acutance dye is often incorporated into photo-sensitive compositions. To be effective, the acutance dye will absorb at the wavelengths at which the photosensitive composition is sensitive. The longer the path length of the light in the layer of light-sensitive composition, the greater the attenuation. Therefore, scattered light is attenuated or absorbed to a larger extent than light which impinges directly on a light-sensitive crystal. As a result, although the overall speed of the composition is reduced slightly, scattered light and other light rays which are liable to produce a blurred image are preferentially absorbed and so the overall definition and sharpness of images produced in the layer are increased.
An acutance dye for use in a dry silver composition is preferably heat labile in the system, that is to say, it is destroyed by the heat development of the photothermographic composition to one or more compounds which are substantially colorless. The exact mechanism of this reaction is not known.
The thermal development process described above and sometimes known as the "dry silver" process is disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075.
Acutance agents for use in thermally-developable photographic materials have been described. In U.S. Pat. No. 3,745,009 there are described antihalation compounds which are energy-decolorizable and suitable for use in thermally-developable photographic materials but are, however, excessively sensitive to heat during production and coating and may decompose prematurely due to ambient heat.
In U.S. Pat. No. 3,769,019 there are disclosed acutance agents which are thermally-decolorizable protonic dyes although color may return spontaneously after a period of time.
In U.S. Pat. Nos. 3,984,248, 3,988,154 and 3,988,156 there are disclosed thermally stable, photobleachable o-nitro-substituted arylidene dyestuffs of the general formula: ##STR1## wherein k=0, 1 or 2; m=0 or 1; L is a methine or substituted methine group; R is an alkyl, substituted alkyl, alkenyl, aryl or substituted aryl group; Y represents the atoms necessary to complete a mono- or polycyclic aryl group which may be further substituted; and Z represents the atoms necessary to complete a mono- or polycyclic heterocyclic nucleus which may be substituted, the heterocyclic ring containing the nitrogen atom shown being 5 or 6 membered. These dyestuffs may be incorporated in heat developable photographic elements as part of an antihalation layer. Such elements may be exposed to a light pattern, thermally developed to provide a sharp image in the element, and then the element fully exposed to light to decolorize the antihalation compounds. Alternatively the compounds may be incorporated as desensitizers in photosensitive compositions for wet processing. Patentee's compounds have an odd number of carbon atoms in the methine bridging group between the cyclic units. This differs from the 2-membered methine chain of the compounds of the present invention. In addition, patentee's compounds do not include the indole nucleus as the heterocyclic moiety. Patentee's compounds are made by reaction of a heterocyclic salt and a nitrosubstituted compound. Such a preparation will not lead to the compounds of the present invention.
Certain nitrostyryl dyes are known to act as desensitizing dyes. In U.S. Pat. Nos. 2,953,561 and 3,023,102 styryl dye bases are utilized as desensitizers for the production of direct positive photographic emulsions. These dye bases are of the general formula: ##STR2## wherein R is hydrogen, halogen, hydroxy, amino, nitro, etc.; n is an integer of from 1 to 2; m is an integer of from 1 to 2; A is a methine group and Z represents the non-metallic atoms necessary to complete a heterocyclic nucleus containing 5 to 6 atoms in the ring. Patentee's compounds can include an indolenine nucleus rather than an indole nucleus such as in compounds of the present invention.
In U.S. Pat. No. 2,541,472, styryl bases in which the nitrogen atom is quaternized are used as desensitizing dyes.
U.S. Pat. No. 4,033,948 discloses certain members of the class of odd-numbered methine arylidene dye bases useful as acutance agents in thermally-developable photosensitive compositions.
Assignee's three copending applications, Ser. No. 964,479 (Lea and Reece), Ser. No. 964,480 (Brown), both filed Nov. 29, 1978, and Ser. No. 98,386 (Lea and Burrows), filed Nov. 29, 1979, likewise disclose acutance agents for use in thermally developable photosensitive compositions.
One aspect of this invention provides dry silver compositions containing acutance dyes, which absorb light in at least some of those wavelengths to which the composition is sensitive and which are rendered colorless upon heat development of the dry silver composition.
In another aspect the invention provides novel compounds suitable for use as acutance dyes in photothermographic compositions such as in sheet materials, and a method for their preparation.
In still another aspect of this invention acutance agents for use in dry processing silver reproduction materials are provided, particularly acutance agents which do not tend to desensitize the photosensitive compounds in said reproduction materials.